The effect of strain on the superconducting properties of triniobium tin (Nb.sub.3 Sn) has been studied for nearly thirty years. It is well known in the art that compressive strains in triniobium tin protect the brittle material from damage that may be encountered during handling, coiling, or from electromagnetic field forces in an energized magnet.
Compressive strains are usually imparted to the triniobium tin by the materials that encase the superconductor. Such materials include bronze in bronze-processed multifilamentary wire or copper foil in laminated liquid-phase diffusion-processed trinibium tin foil. The compressive strain is caused by the difference in thermal expansion coefficients of the copper or bronze cladding and the triniobium tin superconductor. This difference leads to compression of the triniobium tin when the composite is cooled from the processing temperature.
Most triniobium tin superconductor strain-effect research has been performed at high fields on mono- and multi- filamentary bronze-processed wire, which is formed by solid-phase diffusion process.
For instance, a strain scaling relation has been developed by J. W. Ekin, "Strain Scaling Law and the Prediction of Uniaxial and Bending Strain Effects in Multifilamentary A15 Superconductors," Multifilamentary A15 Superconductors, M. Suenaga and A. F. Clark, ed., Plenum Press, New York, 1980, 187-203. Ekin's relation predicts the effect of strain on the critical current, I.sub.c, based on the axial strain dependence of the magnetic critical field, B.sub.c2, for multifilamentary superconductors.
Ekin's paper explains that when uniaxial strain is applied to a multifilamentary triniobium tin conductor, the critical current density, J.sub.c, typically increases to a maximum value J.sub.cm at some strain and then decreases under further uniaxial tension. This maximum in critical current density arises from compressive prestress which the bronze matrix exerts on the triniobium tin reaction layer because of the difference in thermal contraction between the two materials on cooldown after the reaction heat treatment. It is thought that the maximum occurs where the triniobium tin experiences the smallest magnitude of intrinsic strain.
There is a need to develop a method to enhance the critical current of triniobium tin superconducting articles formed by liquid-phase and solid-phase diffusion processes.